SP6: rebuild Apollo Federation v2 (v3 north star)

[rawkode]

Background

TNQ-3 surfaced that docs/v3-decisions.md and docs/spec-gap-analysis.md declared opposite north stars for GraphQL. Escalated to @rawkode; resolution: rebuild federation per the spec. Current kernel-owned-GraphQL is the interim shape and stays as the shipping contract for kernel-mandatory fields (auth / identity / instance config).

Docs alignment landed in #202.

Scope

Implement spec §A7 — Apollo Federation v2 — per docs/spec-gap-analysis.md. High-level shape:

1. Spec → plan cycle of its own (matches the existing spec-driven flow). Includes a build-time spike:
   • planner-as-crate (Hive Router embeddable shape)
   • plan-IR-drives-executor (custom plan executor sees the planner's IR, not a serialized query)
   • join-spec dialect compatibility (which subset of Apollo's join-spec we emit and consume)
2. WIT contract additions
   • graphql-resolve host export so each extension can answer subgraph queries
   • Manifest field for the extension's subgraph SDL
3. Composition + planning at runtime
   • graphql-composition (Grafbase) for SDL composition
   • Hive Router as query planner
   • Custom WASM plan executor that drives the planner's plan into per-extension graphql-resolve calls (no apollo-federation/harmonizer at runtime, per spec)
4. Migration of kernel-owned GraphQL fields that are extension-owned in the federated world (per-extension subgraphs absorb their concerns; kernel keeps the small set it must own)
5. First-party extension subgraph rollout (issues, epics, pull-requests, checks, docs, workspace-home) — each extension ships SDL + handles its graphql-resolve
6. Documentation rewrite — GOAL.md and SPEC.md updated end-to-end (initial alignment in docs: align north star on SP6 Apollo Federation v2 rebuild (TNQ-3 P1) #202 is a placeholder; the full rewrite happens with SP6 build)
7. Test coverage — composition tests, planner tests, executor tests, per-extension subgraph tests, end-to-end federated query tests

Out of scope (separate work items)

• SSH transport (spec §A9 §SP2) — already tracked
• Argon2 PATs / admin-repo identity model (spec §A5) — already tracked
• Per-extension libSQL migration (spec §A3) — separate decision

References

• spec-gap-analysis.md §A7 — chosen design
• v3-decisions.md — current shape vs north star
• docs/wit-platform-design.md — WIT host import shape
• GOAL.md, SPEC.md — current allowances for kernel-owned fields